Introduce a new `getAssetBalance` to RPC's `Server` to fetch any asset balance

[Shaptic]

Is your feature request related to a problem? Please describe.
Retrieving balances is unintuitive for users, because contract balances are represented in contract storage of the SAC, whereas account balances are represented in a trustline. While there are methods for each of these (getSACBalance and getTrustline, respectively), unifying them into a single method that works on both would be a better developer experience.

Describe the solution you'd like
A new method on rpc.Server:

class Server {
  public async getAssetBalance(for: string, of: Asset): Promise<Api.BalanceEntry>
}

where for is either a contract ID (C...), in which case you'd fetch the SAC's balance entry (via getSACBalance), or an account ID (G...), in which case you'd fetch the trustline entry (via getTrustline).

Describe alternatives you've considered
You can use the individual methods for each address type.

Additional context
#1282, discussion